What is Magnetometer?

Magnetometers are widely used for measuring the Earth's magnetic field and in geophysical surveys. to detect magnetic anomalies of various types. 

Proton magnetometer:

It also known as the proton precession magnetometer (PPM), uses the principle of Earth's field nuclear magnetic resonance (EFNMR) to measure very small variations in the Earth's magnetic field. allowing ferrous objects on land and at sea to be detected.

Working Principle:

that protons in all atoms are spinning on an axis aligned with the magnetic field.  The frequency at which the protons precess is directly proportional to the strength of the earth's magnetic field.